Cryogenic cooling is essential for the operation of sensitive infrared imaging devices, infrared interferometers, extremely low noise electronic circuits, and other apparatus used on satellites and planetary probes. A refrigeration system suitable for cooling such apparatus represents a significant technological problem, particularly when the cryogenic refrigeration system must function for an extended period of time without possibility of repair. Although a dewar flask containing an expendable cryogenic fluid might provide a short-term source of low temperature coolant, this solution is not practical for long duration missions. To provide such cooling for an extended period, a dependable closed loop cryogenic cooling system is required.
Conventional mechanical cryogenic refrigeration systems cannot provide the reliability essential for operation where repair is impractical. Attempts to extend the life of conventional mechanical cryogenic refrigeration systems by providing redundant critical components and using the highest quality materials have achieved only limited success.
A prospective solution to this problem appears in commonly assigned U.S. Pat. No. 4,671,080. In the closed cryogenic cooling systems disclosed in this patent, an electrochemical pump is used to provide a pressurized gas stream of hydrogen (or oxygen) to a high pressure flow path. One or more heat exchangers are provided in heat transfer relationship with the pressurized flow path for cooling the gas below its inversion temperature (the highest temperature at which throttling will reduce its temperature). The cooled, pressurized gas expands through a Joule-Thomson flow restrictor into a load heat exchanger, providing cryogenic cooling to components that are attached thereto. The gas follows a low pressure flow path, returning in a closed loop to the electrochemical pump. Before completing the cycle, it passes through a regenerative heat exchanger, cooling the high pressure gas. Since the cryogenic cooling system disclosed in this patent uses no moving parts, it potentially has a much longer usable life than a mechanical cryogenic cooling system.
Alternate approaches to constructing an electrochemical pump for either hydrogen or for oxygen are disclosed in the above-referenced patent. Generally, such a pump comprises a solid electrolyte membrane sandwiched between a porous anode and cathode. The membrane is operative to pump either hydrogen ions (protons) or hydronium ions(H.sub.3 O.sup.+). With respect to the latter, water is formed at the cathode as a by-product that must be separated from the pressurized hydrogen. A similar electrochemical pump for oxygen, employing an ionic membrane conductor for oxygen ions is also disclosed in the patent.
In a cryogenic cooling system employing hydrogen as the working fluid, like that of U.S. Pat. No. 4,671,080, it is necessary to precool the compressed hydrogen gas below its inversion temperature (160 K.) prior to throttling it through a Joule-Thomson valve. In fact, the efficiency of the refrigeration cycle is greatly improved as the gas is further cooled below its inversion temperature. Regenerative cooling of the pressurized gas using the cool low pressure gas reduces the cooling available for the load, i.e., does not improve the refrigeration cycle efficiency. Precooling using sun shielded radiating surfaces exposed to the vacuum of space would require unacceptably large radiators.
Accordingly, it is an object of the present invention to provide a closed cryogenic cooling system which includes means for precooling a primary cryogenic working fluid well below its inversion temperature. It is a further object of the invention to improve the refrigeration cycle efficiency of a closed cryogenic cooling system, as compared to the prior art.
These and other objects of the present invention will be apparent from the attached drawings and the disclosure of the preferred embodiments that follows.